Theories of Infant Development

[Pages:51]Chapter 2

Theories of Infant Development

CHAPTER OUTLINE AND OVERVIEW

Biological Approaches

Learning Theories Cognitive Theories Systems Theories Clinical Theories

Experiential Exercises

In what way has the evolutionary history of the human species contributed to infant behavior and development?Does genetic inheritance account for behavioral and psychological differences between infants?

How do infants learn?What are the conditions most conducive to infant learning?

What are the developmental origins of perceiving, remembering, thinking, and speaking? Are infants intelligent before they can speak?

How are infants connected to the world of people and objects around them? What is the role of the parent-infant relationship in early development?

What has therapy with adults and children revealed about how infancy contributes to psychological development? What are the therapeutic effects of reexperiencing normal infant behavior and development?

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People acquire a personal theory about infants in the course of working with infants or raising one of their own. A scientific theory differs from personal theories in several ways. A scientific theory is a set of concepts that explains the observable world with structures, processes, or mechanisms that are presumed to exist but that cannot be observed directly.

1. A scientific theory helps to organize observations derived from systematic research, using accepted methods of observation and assessment.

2. A scientific theory is phrased in terms of general principles that can be applied to specific research findings and applications.

3. A scientific theory should accurately predict future observations in a majority of cases. A theory whose predictions are not confirmed should be changed or abandoned.

Scientific theories of human development focus on describing and predicting the ways in which children change over time and the origins of individual differences. Why does one child become adept at language skills from an early age, for example, while another is slow to pick up these abilities?

In this chapter, we shall examine a variety of theories of human development that have been applied to infancy: biological, learning, cognitive, systems, and clinical theories. The main principles and concepts of each theory are described, the historical trends within each are reviewed, and the contributions of the theory to contemporary research are outlined. Finally, the main limitations of the theory are discussed.

BIOLOGICAL APPROACHES

Biological approaches are based on the work of Charles Darwin, who theorized that differences between species and between individuals are shaped by whether or not the individual has the ability to survive long enough to reproduce. In a process called natural selection, the individuals who can successfully adapt to the environment will live long enough to reproduce and pass some of their characteristics down to the next generation. Thus, the environment influences which types of characteristics will survive and continue to evolve (Darwin, 1859).

Part of what gets passed down between generations is the genetic code. In sexually reproducing species, each parent passes half of his or her genetic code to the offspring (see Chapter 3). The genetic code is a set of chemical instructions for producing proteins in the nucleus of a living cell. This raw genetic code, made up of large molecules of deoxyribonucleic acid (DNA), is called the genotype. Every cell in a person's body contains the same exact genotype. In order for different cells to take on different functions--such as the neural, skeletal, muscular, and other tissues that underlie mental and behavioral abilities--the environment of the cell and of the organism as a whole affects the actions of the genotype. This occurs via the epigenome, a set of biochemical markers that are respon-

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sive to the environment and that literally turn on or turn off the actions of particular genes within each cell (see Chapter 3).

The other part of what gets passed down between generations is a particular type of environment, including the physical and social environment. Children inherit not only the parent's genes but also the parent's environment. Prenatal development, for example, could not occur outside of the special environment of the female reproductive system. The temperature, chemistry, biology, and physics of that environment are just as important for the development of the individual and the evolution of the species as the composition of the DNA. Individual differences in that environment are just as important as individual differences in the DNA in the formation of the next generation (Hern?ndez, Blasi & Bjorklund, 2003; Lickliter, 2008; see Chapter 3).

The outcomes of the genotype-environment interactions--the resultant products--are called phenotypes and include not only tissues but also behaviors, intelligence, and temperament. Thus, it is important to understand that the genotype does not directly determine the phenotype; rather, the genotype determines the opportunities by which the environment may have an influence on the phenotype (Gottlieb, 1991b). Natural selection, then, does not operate on the genetic code directly. Natural selection operates on the phenotypes, the characteristics of the individual in his or her environment. In this section, we will review two biological approaches: behavior ecology theory and behavior genetics.

Behavior Ecology Theory

Species-specific behavior. Behavior ecology theory is the study of behavior from an evolutionary perspective. Drawing upon Darwin's ideas, behavior ecology theory suggests that all animals have species-specific behaviors that evolved through the process of natural selection. Species-specific behaviors are those that are seen in only one species, such as chimpanzee calls or human speech. Although these two behaviors are species-specific, both are examples of a more general function of communication. The selection of a specific form of communication in a species is presumably related to the survival of its members over many generations and the maintenance of a particular environment--including the interindividual social environment--in which that form of communication can flourish.

Another example of a general cross-species behavior related to survival is the attachment bond between parent and infant seen in most mammalian species, but each species shows this in different ways. Mother cats lick and nuzzle their infants, while mother monkeys groom and cuddle their babies and carry them around. All mammals nurse their young, but the styles of nursing differ between species. In dogs, the mother lies on her side while her puppies nurse. The mother does not look at her babies, but she may lick them and smell them.

Critical periods. Another aspect of behavior ecology theory is the prediction that species-specific behaviors are enhanced and modified in specific environments and at

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specific times during the life course. In many animal species, the young are biologically more susceptible to the acquisition of new behavior than older individuals. Often there is a limited period of time early in life during which environmental input can make a difference in later behavior. This period of maximum susceptibility is called a critical period and is a relatively short (compared to the individual's life span) and clearly demarcated period of time in which learning can occur and during which whatever the animal learns has a permanent and irreversible effect.

In some species of birds, for example, attachment of the infant to an adult occurs only during a period of about two hours, several days after hatching. If a gosling follows its mother around during the critical period, it will develop a preference for the mother over other adults and will stay close to her after the critical period ends and for a long time afterward. This learning of preferences for particular adults is called imprinting.

Konrad Lorenz (1965) found that goslings could become imprinted on a number of different objects during this period. Lorenz made goslings imprint on flashlights, electric trains, and even himself. He would walk near the goslings during the critical period, squatting and honking like a mother goose. In his description of these early studies, he wrote, "In the interest of science, I submitted myself literally for hours on end to this ordeal" (Lorenz, 1952, p. 42). For this and other work, Lorenz won a Nobel Prize in 1973. He is one of only three behavioral scientists to have received this honor. The other two--Niko Tinbergen and Karl von Frisch--were also behavior ecologists.

A large number of studies--mostly with mallard ducklings--have been done on imprinting since Lorenz's classic work. One of the main findings is that imprinting occurs only after the ducklings leave the nest and only if they can follow a moving adult duck that is calling to them (Bateson, 1966; Hess, 1959). One study (Dyer, Lickliter, & Gottlieb, 1989) suggests that before imprinting, ducklings may be more sensitive to the visual images of their walking siblings than to the mother duck. Ducklings may initially leave the nest as they see other ducklings doing so. Once this happens, the ducklings will respond as a group to the mother duck's call, follow her as a group, and thus become imprinted. Other environmental factors also affect the imprinting process. Ducklings need to hear their own call prior to hatching. In experiments in which duck embryos were made unable to vocalize, they did not recognize the mother's call after hatching (Gottlieb, 1991a). If quail chicks are exposed to patterned light during the first days after birth, instead of seeing their mothers or siblings, they will not imprint even if they can hear the mother's call (Lickliter & Hellewell, 1992).

In birds, critical periods occur in infancy that set down a behavioral pathway for the remainder of their lives. Cow bird males, for example, can either become aggressive and competitive in finding a single mate in their adult sexual behavior, or they can be egalitarian and choose among multiple mates with no competition. It all depends upon whether, as infants, they hear competitive versus egalitarian adult males singing in their environment, since there is a difference in the singing of these two types of adult male (White et al., 2007). We shall see in Chapter 11 that gender stereotypical play in human children depends in part on the behavior of adults in their environment during the period between 2

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In research by the Harlows, infant monkeys preferred the substitute mother that was covered with a soft cloth, rather than the wire mother to which the food was attached. This study showed that attachment in monkeys is more related to contact comfort than to feeding. In humans, attachment is related to play and communication. Courtesy of the University of Wisconsin Primate Laboratory.

and 4 years of age, particularly the behavior of the adult males (Hern?ndez, Blasi & Bjorklund, 2003).

The picture of imprinting that emerges from these studies is that a series of related environmental events must be tied together. Imprinting involves age mates, locomotion, and auditory and visual perception and cannot be thought of as simple photographic images of the mother printed on the duckling's brain. Imprinting does not occur in humans and other primate species because the infants are too immature to follow their parents around. Human parents, therefore, play a greater role in the mutual maintenance of proximity. The lasting emotional tie that promotes this proximity is called attachment (see Chapter 8). Also, the critical period for human attachment in humans, monkeys and apes is longer and the environmental conditions under which attachment may occur are more complex.

If there is severe deprivation of parental care, the critical period for attachment is easier to observe. Studies of infants reared with little adult interaction in orphanages in Eastern Europe and in the Middle East showed that these children developed severe symptoms of withdrawal and have life-long socioemotional impairments. Infant monkeys reared without adults tend to show similar symptoms: rocking, head banging, extreme fear in the presence of strangers, and an inability to form relationships with other individuals (Harlow & Harlow, 1965; Spitz, 1965).

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Even with these clear deprivations, the situation is more complicated than it may seem. Some of the early orphanage studies have been criticized for confusing the symptoms of malnutrition--common in those institutions--with those predicted for maternal deprivation. Since research that deliberately deprives infants of their mothers is ethically forbidden with humans, researchers have used nonhuman primates for this research. Follow-up studies on monkeys deprived of their mothers early in life showed that some social experiences--in particular, interactions between the deprived monkeys and monkeys younger than themselves--provided when the deprived monkeys were juveniles had the effect of partially reversing the social withdrawal (Suomi & Harlow, 1972). Gorilla infants who are raised by humans develop more aggressive and antisocial behaviors than do gorillas raised with gorilla parents. After spending time in a group of gorilla peers, however, human-reared juvenile gorillas will act more like those who were reared by their own mothers (Meder, 1989). Like the duckling studies, attachment during the critical period of infancy depends upon multiple factors.

Another potential example of a human critical period is the development of language in the first three years of life. Again, severe deprivation is found in the case of a girl named Genie, who was found in 1970 at the age of thirteen after having been isolated in a small room since infancy. Her father, apparently a psychotic who hated children, forced Genie to remain in a closet and refused to let anyone speak to her. Susan Curtiss, a developmentalist who spent many years trying to help Genie recover, reported that Genie learned some language (Curtiss, 1977) and after several years of practice could string up to three words together to make her intentions and thoughts known. But she never seemed to grasp the idea of grammar, and she never learned to ask questions.

The problem with this single case study is that we have no way of knowing whether Genie suffered from some form of brain damage or other impairment early in life. Such an organic deficit might be the real cause of Genie's language retardation. Ethically, we cannot do language deprivation experiments on groups of healthy children.

In summary, critical periods exist for humans but they depend upon a complex network of environmental factors. The clearest example of a critical period in human development is the first six months of prenatal development. Environmental influences--such as maternal nutrition and environmental toxins--can have severe effects on the health, physiological and brain development, and behavior that last for a lifetime. These processes will be reviewed in detail in Chapter 3.

Behavior Genetic Theory

Behavior ecology approaches typically focus on species-wide patterns, but what accounts for differences between individuals? Behavior genetics is the study of possible environmental and genetic explanations for individual differences in behavior and personality characteristics. In doing research in behavior genetics, it is essential to know how individuals are genetically related to each other and whether the environments in which they are raised are similar or different. A typical research strategy is to observe behavior, such as in-

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telligence or temperament (a word researchers use to talk about infant personality; see Chapter 6), in identical compared to fraternal twins. Identical twins have the same set of genes, while fraternal twins share only a portion of their genes. Thus, if identical twins reared at home are more similar to each other than are fraternal twins reared at home, it is possible to conclude that at least some portion of the observed behavior is explained by a genetic contribution, since identical twins have more genes in common than fraternal twins, who are no more related to each other than any two siblings in a family.

Another research approach is to study identical twins who are adopted at birth and raised in different families. They will have the same genes but be reared in different environments. Twins reared together will be more similar than twins reared apart, showing that some of the similarity between identical twins is due to the fact that they are raised in the same shared environment rather than to their genetic similarity. The shared environment of identical twins can be observed because twins look alike and are the same sex, so they are more likely to be treated alike by parents than fraternal twins. Identical twins may be dressed alike and encouraged to spend time together, creating opportunities for mutual imitation and similar experiences. Even in the same home, however, each twin may have somewhat different experiences, called the nonshared environment (Loehlin, 1989; Plomin, 1994).

The study of similarities in characteristics between family members is another approach to behavior genetic research (Lemery & Goldsmith, 1999). This kind of study is useful if a particular trait, such as a suspected genetically caused disease, occurs disproportionately within particular families. Identical twins share 100 percent of their genetic material, while other siblings from the same parents share 50 percent of their genetic material, the same amount shared by parents with their children. Uncles and nephews, aunts and nieces, grandparents and grandchildren, and half siblings share 25 percent of their genetic material. If genetics plays a role for a characteristic, there should be a decreased probability of the occurrence of that characteristic in family members who are more distantly related and an increased probability of occurrence among those who are more closely related.

The heritability of the observed behavior measures the extent to which individual differences in the behavior are due to genetic factors. Heritability is usually expressed in terms of a percentage, the percentage of variability between individuals that can be explained by genetic variability. It varies between 0 and 1.00. Research findings show modest heritability (about 30 percent, on average) for some measures of intelligence, temperamental inhibition, empathy, self-esteem, and the ways in which individuals select, modify, create, or avoid specific features of their environments (Plomin, 1994; Robinson et al., 1992; Scarr, 1993; Zahn-Waxler, Robinson, & Emde, 1992). This means that 30 percent of the differences between people in these characteristics can be explained by genetic variability.

Because heritability is expressed as a percentage, results from behavior genetic research are probabilistic. That is, if you possess a particular set of genes, you have a higher probability of developing a particular kind of behavioral characteristic than a per-

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son without those same genes. The action of the genes is not deterministic, since there are environments that might lessen the impact of those genes on the behavioral outcome (Scarr, 1993; Scarr & McCartney, 1983).

Some genetic predispositions, for example, may never appear in the phenotype because the genotype is never exposed to a "needed" environmental resource at a particular time in life (Turkheimer et al., 2003). Thus, a gosling will never manifest its species-typical attachment behaviors if it is not exposed to an adult goose during the critical period for imprinting. This is an example of a genetic predisposition that is easily influenced by the environment. In this case, environmental variability has a larger probability of predicting individual phenotypes than does genetic variability. In fact, genetic variability usually accounts for only a small proportion of behavioral variability. Many genes, each with a small influence, rather than a single gene, are typically involved in influencing behavior (Plomin, 1990, 1994).

Other genotypes may be less susceptible to variations in the environment. If infants have a color blindness genotype, variability in the amount of exposure to visual stimuli will have little influence on the phenotype of color blindness. In this case, genetic variability between individuals (inheriting or not inheriting color blindness genes) has a larger probability of predicting individual phenotypes than does environmental variability.

No genes and no environments are 100 percent deterministic. Some genetic disorders can be eliminated in the phenotype through surgery, by drug treatments, or by avoiding certain environmental factors (e.g., controlling diabetes through diet). There is also hope that some will be "cured" by as yet undiscovered techniques (see Chapter 3). The increased used of molecular biological methods in behavior genetic research also provides hope for the discovery of particular gene-environment interaction processes that can be more easily regulated to prevent or treat serious physical, behavioral, and psychological disorders (Plomin et al., 2003).

In some cases, genetic variability may predict variability in phenotypes early in life but not later. A stable pattern of temperamental withdrawal tends to be correlated with eye color (withdrawn infants are more represented in the group of blue-eyed infants than would be expected by chance) during the first three years of life. During the preschool period and later, there is no correlation between eye color and temperament (Kagan, Reznick, & Snidman, 1987). These findings suggest that temperamental withdrawal may be genetically based, but after several years, the variability of that phenotype in the population is better predicted by environmental rather than genetic variability. The opposite could also be true: a genetically based skill in language might account for between-infant differences at age three, but not earlier in infancy (Fagard & Jacquet, 1989; Ramsay, 1980).

The relationship between genes and environments is made more complicated because they influence each other. People with a particular genetic predisposition--for temperamental withdrawal, for example--may prefer to be with one or two other people rather than in large groups. Their choice of environments, or preference for particular behaviors, then influences the kinds of skills they can later develop (Karmiloff-Smith, 2007; Scarr & McCartney, 1983).

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